ar71xx: Add build profile for the gl-inet 6416A and 6408A

ar71xx: Add build profile for the gl-inet 6416A and 6408A

Patch to add the buildprofile for the GL-Init-6408A-v1 and the GL-Inet-6416A-v1
Both devices are identical, only difference is one comes with 8MB flash and
the other with 16MB flash

Official website: http://www.gl-inet.com/w/?page_id=241&lang=en
Comprehensive list of specs: https://revspace.nl/GL-Inet

Signed-off-by: Martijn Zilverschoon <martijn@friedzombie.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43462 3c298f89-4303-0410-b956-a3cf2f4a3e73